An inkjet recording method can form an image easily and inexpensively, being thereby utilized in a wide variety of printing fields such as photography, various prints, signs, or color filters. When exclusive paper featuring optimum characteristics for inkjet recording is used, image quality comparable to that of silver halide photography can be realized. However, printing requiring such exclusive paper results in high cost and limited applications.
Over recent years, applications to high-speed printing to nonabsorbable media (vinyl chloride, art paper, or coated paper) have been expanding. However, when an image is formed on such a medium using aqueous inks, there has been produced such a problem that spotting and bleeding (hereinafter referred to as bleed) occur due to aggregation among inks. To improve this image quality degradation, it is known that ink viscosity needs to increase to some extent immediately after printing.
As a technology to improve image quality degradation by increasing viscosity immediately after printing, an inkjet ink containing a water soluble resin neutralized with an amine as a binder resin at 2%-10% in terms of solid is used, whereby bleed is considered to be improved via a method of printing while a recording medium is heated (for example, Patent Document 1). It is described that such ink viscosity increase is specifically markedly exhibited in an ink employing a water soluble organic solvent selected from glycol ethers or 1,2-alkanediols. It is thought that drying accelerates water evaporation, whereby the ratio of a glycol ether or 1,2-alkanediol component in a deposited ink is increased and then the interaction of each resin is enhanced, resulting in an increase in ink viscosity.
Although this method realizes an increase in viscosity after deposition due to heating and the presence of a specific solvent and also bleed is improved to some extent in a nonabsorbable medium, the increasing rate of ink viscosity has been inadequate in high-speed printing. Therefore, to achieve high image quality in high-speed printing, a rapid viscosity increase after ink deposition is a critical challenge.
Further, as another technology, disclosed is a thermal history display dye ink which contains an allochroic dye varying with pH changes and a volatile organic amine; and utilizes a pH decrease via volatilization of an ammonium salt, (for example, Patent Document 2). However, deposition and aggregation of a resin and a pigment utilizing such a pH decrease are not described.
It is described that a hydrolyzable silane compound and an ammonium salt are contained, and the ammonium salt releases an amine or ammonia with water evaporation; and then the action of the residual acid accelerates polycondensation reaction, resulting in enhanced water resistance (for example, Patent Document 3). However, it is not specifically described that a resin having a higher pKa than an acid component formed by dissociation of an amine salt in ink is contained, and no disclosure is made with respect to a technological thought on viscosity increase using deposition and aggregation of a resin and a pigment to improve image quality.
It is described that an ink containing barium hydroxide and an ammonium salt can reduce bleed. A viscosity increase via endothermic reaction of such barium hydroxide and an ammonium salt is utilized, but it is not specifically described that a resin is contained (for example, Patent Document 4).
It is described that a treating solution for changing pH is treated to an ink which includes positively or negatively charged water insoluble particles A and water insoluble particles B having both of cationic group and anionic group and changes c potential by changing of surrounding pH environment, whereby aggregation rate is accelerated by utilizing polarity inversion (for example, Patent Documents 5). However, the treating solution for changing pH is necessary, and also required are a large size printer and a complicate process.